Method of applying electrical insulation



April 9, 1968 D. E. WEYEF; 3,377,414

METHOD OF APPLYING ELECTRICAL INSULATION Filed Oct. 5, 1964 II S n 1 4 INVENTOR. aona/a f. Weyer BY Wm M ATTORNEY United States Patent M 3,377,414 METHOD OF APPLYING ELECTRICAL INSULATION Donald E. Weyer, Midland, Mich., assignor to Dow Corning Corporation, Midland, Mich., a corporation of Michigan Filed Oct. 5, 1964, Ser. No. 401,537 5 Claims. (Cl. 264-435) ABSTRACT OF THE DISCLOSURE Method of electrically insulating electrical devices by both impregnation and encapsulation of the device with insulating material. A cup-shaped container is formed by inserting the device into a tube of shrinkable insulating material, which is then caused to shrink tightly around the bottom portion of the device. This cup is then filled with a liquid insulation suitable for impregnating 'the device and the open end of the cup is then caused to shrink tightly around the portion of the device not previously encapsulated.

The present invention relates to insulation techniques and, more particularly, to methods of electrically insulating electrical devices wherein it is desirable for both impregnation and encapsulation of the device with insulating material.

Flyback transformers for television use have in the past been insulated with various types of high dielectric materials, wax being one of the most widely used. The coils of the tranfsormer were wax impregnated and a band of wax was commonly placed around the high voltage coil. Paper was sometimes used with the wax. While this type of insulation served well with monochrome television, the advent of color necessitated higher voltages in the fiyback circuit. With the use of higher voltage, problems in insulating the transformers have developed.

.ter insulation is a necessity.

A primary object is to provide an improved economical insulating technique for high voltage devices.

A further object of the present invention, therefore, is the provision of a high voltage device having improved insulation.

In accordance with these and other objects there is provided by the present invention a technique for electrically insulating electrical devices in a novel manner which elim- Iinates prior art disadvantages and provides a technique easily adaptable to economical mass production. Although the technique herein described was developed, particularly for use in high voltage applications, it may be applied to other types of devices as well.

Other objects and many attendant advantages of the' present invention will become apparent to those skilled in the art by a consideration of the following detailed description when read in conjunction with accompanying drawings wherein:

FIG. 1 is a somewhat diagrammatic view in perspective "illustrating a first portion of the method of the present invention and showing a fiyback transformer in place in a tube of heat shrinkable material, a portion of the tube being shown broken away to reveal the transformer;

3,377,414 Patented Apr. 9, 1968 FIG. 2 is a partially diagrammatic cross-sectional view illustrating a second portion of the method of the present invention;

FIG. 3 is a partially diagrammatic sectional view showing details of the completed insulation; and

FIG. 4 is a view in perspective of a completed transformer insulated in accordance with the method of the present invention.

Referring now to the drawings wherein like reference numerals designate like or corresponding parts throughout the several views, there is shown in FIG. 1, a television fiyback transformer 11 disposed within a length of shrinkable tubing 12. The fiyback transformer is no part of the present invention, and has been chosen merely for purposes of illustration, it being realized that other and different types of electrical devices may be insulated in the manner hereinafter described.

The shrinkable tubing is preferably of a heat shrinkable material such as those described in Belgian Patent 609,815, or those described in US. patent application Ser. No. 296,486, filed July 22, 1963. Such material has a shrink temperature at which it will shrink to one-third or less of its original size. Alternatively, other methods of expansion and contraction of elastic material may be utilized. Of primary importance in choice of materials is the necessity that the material have a high dielectric strength, and in general, that it be a sufficiently .good electrical insulator for the particular application. The material must also have sufficient physical strength for the particular application and must retain its physical and electrical properties throughout the temperature range in which it is to be used.

The transformer of FIG. 1, is shown with low voltage leads 13, a high voltage lead 14 extending parallel to the axis of the coil, and a second high voltage terminal 15 on the outer periphery of the device. The tube 12 has an initial inner diameter slightly larger than the greatest diameter of the transformer or other device being insulated to facilitate insertion of the device into the tube.

After the device is positioned in the tube 12, one end of the tube is caused to shrink to fit tightly the periphery of the corresponding end of the device. In the case of heat shrinkable material this is accomplished by application of the necessary amount of heat to one end of the tube to raise the temperature of said end to or above the heat shrink temperature of the tube material, while the other end remains at a lower temperature. The temperature required depends on the particular type of material used as explained in the aforementioned Belgian Patent and US. patent application.

The tube then is in the shape of an open cup as shown in FIG. 2, and is sealed against one end portion of the transformer. Insulation impregnant 16 in liquid form is then poured into the open end of the cup to a level suflicient to cover the portion of the device which is to be insulated, and the device may be placed in a vacuum as indicated by the chamber 17 and attached vacuum pump 18 to allow'the impregnant to fill all voids in the device. If the impregnant is sufficiently fiowable the vacuum may be dispensed with. The insulating impregnant per se also forms no part of the present invention, it being understood that any known type, such as commercially available liquid silicone materials or paraffin, may be used. It must, of course, have proper dielectric strength and thermal limits for the particular application and a low enough viscosity for impregnating voids in the type of device being insulated. Obviously, a more viscous material may be used in devices where openings are large.

After the device has been impregnated by the liquid impregnant, the device is removed from the vacuum and the excess impregnant is poured olf. Again, assuming that tube 12 is of heat shrinkable material, heat is then applied to the whole device causing the tube to shrink to con-form to the periphery of the device 11, and bond itself to the impregnant as shown in FIG. 3. Obviously, if the tube is other than heat shrinkable, other shrinking means are applied for the same purpose.

, FIG. 4 shows the completed device. The terminal 15 at the outer periphery of the device has been exposed by present invention this may be accomplished by placing the device with a tube of heat shrinkable insulation material surrounding it in a vat which is filled with heated liquid impregnant. The vat may be evacuated, if desired, to aid impregnation. The temperature of the liquid is at least as high as the shrink temperature of the tube material. As soon as the tube is heated by immersion in the hot liquid, either by filling the vat containing the devices, or by immersing the devices into liquid in a vat, the tube is caused to shrink into conformance with the periphery of the device and acts to seal impregnant inside.

It will further be realized that the tube need not be as long as the device if only a portion of the device is to be insulated. For example, in flyback transformers it is often necessary to insulate only the outermost periphery of the high voltage coil with heavy insulation. In such instances, a short tube of shrinkable insulation material may be supported around the high voltage coil and shrunk over its periphery during impregnation.

Other modifications and variations of the invention will become obvious to those skilled in the art. Accordingly, it is to be understood that within the scope of theappended claims, the invention may be practiced otherwise than as specifically described.

That which is claimed is:

1. A method of insulating an electrical device comprising inserting said device into a tube of heat shrinkable insulating material, said tube initially having an internal diameter slightly larger than the largest diameter of said device,

heating one end of said tube to cause said tube to shrink into tight fitting relationship with the corresponding end of said device, while leaving the other end of said tube open,

filling the open end of said tube with insulating impregnant in liquid form to a level sufiicient to cover the portion of said device to be insulated,

placing said device and impregnant in a vacum to cause said impregnant to eliminate any voids in said device, draining the remaining impregnant from said device, and

heating the remainder of said tube to cause said tube to shrink into tight fitting relationship with said device.

2. A method of insulating an electrical device comprising inserting said device into a tube of shrinkable insulating material, said tube initially having an internal diameter slightly larger than the largest diameter of said device,

shrinking one end of said tube to cause said tube to fit tightly a corresponding portion of said device, while leaving the other end of said tube open,

filling the open end of said tube with liquid impregnant to a level suflicient to cover the portion of said device to be insulated and allowing said impregnant to fill any voids in said device,

draining any remaining impregnant from said device,

and

shrinking the remainder of said tube to fit tightly the impregnated area of said device.

3. A method of insulating an electrical device comprising,

inserting said device into a tube of shrinkable insulating material,

shrinking one end of said tube to fit tightly a corresponding portion of said device while leavingthe other end of the tube open,

filling the open end of said tube with liquid insulating impregnant to a level sufficient to cover the portion of said device to be insulated and allowing said impregnant to fill any voids in said device, draining any remaining impregnant from said device, and

shrinking the remainder of said tube to conform with the surface of the impregnated portion of said device.

4. A method of insulating a transformer comprising,

inserting said transformer into a tube of heat shrinkable insulating material, said tube having an internal diameter slightly larger than the largest diameter of said transformer and said material having a predetermined shrink temperature,

heating one end of said tube above its shrink temperature to cause said tube to shrink into tight fitting relationship with the corresponding end of said transformer, while leaving the other end of said tube open,

filling the other end of said tube with insulating impregnant to a level sufficient to cover at least the portion of said transformer to be insulated and a1 lowing said impregnant to fill any voids in said transformer,

draining any remaining impregnant from said transformer,

and heating the remainder of said tube above its shrink temperature to cause said tube to shrink into tight relationship with the exterior of said transformer.

5. A method of insulating an electrical device comprising:

first insenting said device into a tube of heat shrinkable insulating material, the material of said tube having a predetermined shrink temperature, and said tube initially having an internal diameter slightly larger than the largest diameter of said device to be insulated;

then placing said device and tube into a vat, which vat is filled with liquid insulating impregnant having a temperature at least as high as the predetermined shrink temperature of said tube, thereby causing said tube to be immersed in said impregnant and causing said impregnant to enter and fill said tube and to cover at least that :portion of said device to be insulated,

whereby any voids in said device are filled by said impregnant and the temperature of said tube is gradually raised to said predetermined shrink temperature by transference of heat from said impregnant to said tube, thereby causing said tube to shrink and squeeze the remaining impregnant from said tube, while said tube conforms and bonds itself to the external periphery of said device.

References Cited UNITED STATES PATENTS Re. 25,624 7/1964 Stahl 264-230 3,250,837 5/1966 Inoue 264---342 X 3,214,719 10/1965 Turner I 264-272 X ROBERT F. WHITE, Primary Examiner.

R. B. MOFFIT, J. H. SILBAUGH,

Assistant Examiners. 

